Highlights
We present the fourth release of a combined measurement and modeling system that keeps track of the emissions ("sources") and removal ("sinks") of atmospheric CO2 globally from January 2000 through December 2008. CarbonTracker 2009 (released on 9 November 2009) incorporates several improvements over the previous release.

We have added several observation sites within North America. Quasi-continuous measurements at Lac Labiche, Alberta, Canada and East Trout Lake, Saskatchewan, Canada have been provided by Environment Canada (EC). ESRL also adds quasi-continuous measurements from towers in Boulder, Colorado; Beech Island, South Carolina in collaboration with Savannah River National Laboratory and University of Georgia; Shenandoah National Park, Virginia in collaboration with University of Virginia, Department of Environmental Sciences; and Walnut Grove, California in collaboration with DOE Environmental Energy Technologies Division at Lawrence Berkeley National Laboratory.

Global resolution of the TM5 atmospheric transport
model used in CarbonTracker has improved from 6°
longitude x 4° latitude to 3° x 2°.

First-guess fluxes from the terrestrial biosphere
model and the air-sea flux model are now available in
binary format, so that the changes that CarbonTracker
estimates can be analyzed by interested users.

Optimized fluxes are presented at two different
scales: the 1° longitude by 1° latitude
distribution used in previous CarbonTracker releases,
and a distribution at "ecoregion" scale. Ecoregions,
described in detail here, are the native
scale at which CarbonTracker performs its optimization.
Showing fluxes at this scale--without the within-region
patterning derived from underlying models--is in some
ways a more direct way of presenting CarbonTracker's
results.

Figure 1: The long term mean biological uptake

Figure 2: The long term mean fossil fuel emissons

Estimates of CO2 sources and sinks
From 2001 through 2008 ecosystems in North America have
been a net sink of 0.59 ± 0.55 PgC yr-1 (1 Petagram Carbon equals 1015 gC, or 1 billion metric ton C, or
3.67 billion metric ton CO2).
This natural sink offsets about one-third of the
emissions of 1.9 PgC yr-1 from
the burning of fossil fuels in the U.S.A., Canada and
Mexico combined. Our estimates include sub-continental
patterns of sources and sinks coupled to the distribution
of dominant ecosystem types across the continent (see
Figures 1 and 2). The sinks are mainly located in the
agricultural regions of the Midwest (36%), deciduous
forests along the East Coast (33%), and boreal coniferous
forests (17%). There also appears to be substantial year
to year variation of the sink, with the net natural
emissions to the atmosphere ranging from -0.2 to -0.8 PgC
yr-1. These variations are
strongly correlated with large scale temperature and
moisture variations. We estimated the lowest net annual
uptake by ecosystems in 2002, when there were widespread
drought conditions through most of the West. In
contrast, our observing system did not detect an effect
from the 2007 drought in the Southeast. This is likely
due to lack of coverage of the area (Figure 3) in our
current observing network. The estimates are optimally
consistent with measurements of ~26,000 flask samples of
air from 72 sites across the world, ~20,000 daily
averages of continuously measured CO2 at 9 sites (6 in North America, plus
observatories at Mauna Loa, Hawaii; South Pole; and
American Samoa), and ~22,000 daily averages from towers
at 13 locations within the continent (see Figures 3 and
4). Eight of these towers sample air from heights more
than 100m above ground level.

Figure 3: North American sites only

Word of caution about the biological flux maps
Figure 1 shows 1° x 1° detail for estimated
fluxes. With the present observing network, the detailed
1° x 1° degree fluxes should not be interpreted as
quantitatively meaningful for each block. To spread the
influence of sparse observing sites we make the assumption
that large
ecosystem regions respond in the same way to
variations of temperature and light. However, temperature
and light are not uniform in an entire region, and thus
the same response function does not produce a uniform flux
over the region. Thus we caution that the spatial detail
is only predicted by CarbonTracker based on the assumption
of large-scale ecosystem coherence, but has not been
verified by observations.

Calculated time-dependent CO2 fields throughout the atmosphere
A "byproduct" of the data assimilation system, once
sources and sinks have been estimated, is that the mole fraction of
CO2 is calculated everywhere in
the model domain and over the entire 2000-2008 time
period, based on the optimized source and sink estimates.
As a check on model transport properties, calculated
CO2 mole fractions are regularly
compared with measurements of ~27,000 air samples taken by
NOAA/ESRL at 26 aircraft sites, which are not used in the
estimation of sources and sinks. Column averages of the
CO2 mole fraction have been
calculated as well, and they can be compared to satellite
measurements of the same quantity when the averaging is
done in the same way as for the satellite results.

Figure 4: All observation sites used in CarbonTracker

Uncertainties
It is important to note that at this time the uncertainty
estimates for the sources/sinks are themselves quite
uncertain. They have been derived from the mathematics
of the data assimilation system, which require several
"educated guesses" for initial uncertainty estimates.
The paper describing CarbonTracker (Peters et al. (2007),
Proc. Nat. Acad. Sci. vol. 104, p. 18925-18930)
presents different uncertainty estimates based on the
sensitivity of the results to 14 alternative yet
plausible ways to construct the CarbonTracker system.
For example, the 14 realizations produce a range of the
net annual mean terrestrial emissions in North America of
-0.40 to -1.01 PgC -1 (negative
emissions indicate a sink). The procedure is described
in the Supporting Information Appendix to that paper,
which is freely downloadable from the PNAS web site.
Furthermore, the estimates do not take into account
several additional factors noted below. The calculation
was set up for sources and sinks to slowly revert, in the
absence of observational data, to "first guesses" of net
ecosystem exchange, which are close to zero on an annual
basis. This set-up may result in a bias. Also due to
the sparseness of measurements, we have had to assume
coherence of ecosystem processes over large distances,
giving existing observations perhaps an undue amount of
weight. The process model for terrestrial photosynthesis
and respiration was very "basic", and will likely be
greatly improved in future releases of CarbonTracker.
Easily the largest single annual mean source of CO2 is emissions from fossil fuel
burning, which are currently not estimated by
CarbonTracker. We use estimates from emissions
inventories (economic accounting) and subtract those from
the total sources derived by CarbonTracker. A small
relative error in the inventories would thus translate
into a larger relative error in the annual mean ecosystem
sources/sinks that have smaller magnitudes. We expect to
add a process model of fossil fuel combustion in future
releases of CarbonTracker. Finally, additional
measurement sites are expected to lead to the greatest
improvements, especially to more credible and specific
source/sink results at smaller spatial scales.